In the oil and gas recovery industry there are many operations that are done to a well during its lifetime. All oil or gas wells have equipment associated therein which requires regular maintenance. To carry out this maintenance, "plugs" have to be installed at various depths in the well by wireline operations to make the well safe.
All plugs have to be designed to hold pressure differentials in both directions, firstly pressure from below, that is well build up pressure, and secondly pressure from above, that is equipment above the plug, will be pressure tested by pressurizing against the plug. It is essential however that the pressure is equalized around the plug prior to retrieving the plug. To accomplish this equalization, conventional plugs require a wireline run in the well to manipulate the plug's "equalization device". Once equalized a second wireline run is required to retrieve the plug.
One attempt at overcoming the problem of having to make two wireline runs has been to incorporate a shear pin into the internal mechanism of the plug so that when the pressure on the upper section of the plug is for example, 200 psi greater than the pressure below the plug the shear pin breaks and the mechanism is released to enable equalization to take place. However, this solution has the problem that there are very wide tolerance margins in the shear pins which means that it may be necessary to use a pressure differential which is substantially greater than 200 psi in order to shear the pin. In addition, in order to shear the pin it may be necessary to exceed the safe working pressure of the well pipe and christmas tree located above the plug. This is often the case especially if the seals on the christmas tree and well pipe have been tested to their operational maximum limit and this is similar to the well build up pressure below the plug.
A further problem with the shear pin mechanism may occur if a live well is plugged deep with such a mechanism while drilling another well. If the well being drilled accidentally breaches the plugged well, drilling mud will enter the plugged well above the plug. The weight of the drilling plug falling onto the plug with a shear pin mechanism in the live well could be sufficient to shear the pin accidentally. Clearly such a situation is undesirable during drilling as the pressure below the plug is released into both wells when the pin shears.